4 Software architectureThe design process for identifying the sub-systems making up a system and the framework for sub-system control and communication is architectural designThe output of this design process is a description of the software architecture

5 Architectural design An early stage of the system design processRepresents the link between specification and design processesOften carried out in parallel with some specification activitiesIt involves identifying major system components and their communications

6 Advantages of explicit architectureStakeholder communicationArchitecture may be used as a focus of discussion by system stakeholdersSystem analysisMeans that analysis of whether the system can meet its non-functional requirements is possibleLarge-scale reuseThe architecture may be reusable across a range of systems

7 Architectural design processSystem structuringThe system is decomposed into several principal sub-systems and communications between these sub-systems are identifiedControl modellingA model of the control relationships between the different parts of the system is establishedModular decompositionThe identified sub-systems are decomposed into modules

8 Sub-systems and modulesA sub-system is a system in its own right whose operation is independent of the services provided by other sub-systems.A module is a system component that provides services to other components but would not normally be considered as a separate system

9 Architectural modelsDifferent architectural models may be produced during the design processEach model presents different perspectives on the architecture

10 Architectural modelsStatic structural model that shows the major system componentsDynamic process model that shows the process structure of the systemInterface model that defines sub-system interfacesRelationships model such as a data-flow model

11 Architectural stylesThe architectural model of a system may conform to a generic architectural model or styleAn awareness of these styles can simplify the problem of defining system architecturesHowever, most large systems are heterogeneous and do not follow a single architectural style

13 System structuringConcerned with decomposing the system into interacting sub-systemsThe architectural design is normally expressed as a block diagram presenting an overview of the system structureMore specific models showing how sub-systems share data, are distributed and interface with each other may also be developed

15 The repository modelSub-systems must exchange data. This may be done in two ways:Shared data is held in a central database or repository and may be accessed by all sub-systemsEach sub-system maintains its own database and passes data explicitly to other sub-systemsWhen large amounts of data are to be shared, the repository model of sharing is most commonly used

17 Repository model characteristicsAdvantagesEfficient way to share large amounts of dataSub-systems need not be concerned with how data is produced Centralised management e.g. backup, security, etc.Sharing model is published as the repository schemaDisadvantagesSub-systems must agree on a repository data model. Inevitably a compromiseData evolution is difficult and expensiveNo scope for specific management policiesDifficult to distribute efficiently

18 Client-server architectureDistributed system model which shows how data and processing is distributed across a range of componentsSet of stand-alone servers which provide specific services such as printing, data management, etc.Set of clients which call on these servicesNetwork which allows clients to access servers

20 Client-server characteristicsAdvantagesDistribution of data is straightforwardMakes effective use of networked systems. May require cheaper hardwareEasy to add new servers or upgrade existing serversDisadvantagesNo shared data model so sub-systems use different data organisation. data interchange may be inefficientRedundant management in each serverNo central register of names and services - it may be hard to find out what servers and services are available

21 Abstract machine modelUsed to model the interfacing of sub-systemsOrganises the system into a set of layers (or abstract machines) each of which provide a set of servicesSupports the incremental development of sub-systems in different layers. When a layer interface changes, only the adjacent layer is affectedHowever, often difficult to structure systems in this way

23 Control modelsAre concerned with the control flow between sub-systems. Distinct from the system decomposition modelCentralised controlOne sub-system has overall responsibility for control and starts and stops other sub-systemsEvent-based controlEach sub-system can respond to externally generated events from other sub-systems or the system’s environment

24 Centralised controlA control sub-system takes responsibility for managing the execution of other sub-systemsCall-return modelTop-down subroutine model where control starts at the top of a subroutine hierarchy and moves downwards. Applicable to sequential systemsManager modelApplicable to concurrent systems. One system component controls the stopping, starting and coordination of other system processes. Can be implemented in sequential systems as a case statement

27 Event-driven systemsDriven by externally generated events where the timing of the event is outwith the control of the sub-systems which process the eventTwo principal event-driven modelsBroadcast models. An event is broadcast to all sub-systems. Any sub-system which can handle the event may do soInterrupt-driven models. Used in real-time systems where interrupts are detected by an interrupt handler and passed to some other component for processingOther event driven models include spreadsheets and production systems

28 Broadcast modelEffective in integrating sub-systems on different computers in a networkSub-systems register an interest in specific events. When these occur, control is transferred to the sub-system which can handle the eventControl policy is not embedded in the event and message handler. Sub-systems decide on events of interest to themHowever, sub-systems don’t know if or when an event will be handled

30 Interrupt-driven systemsUsed in real-time systems where fast response to an event is essentialThere are known interrupt types with a handler defined for each typeEach type is associated with a memory location and a hardware switch causes transfer to its handlerAllows fast response but complex to program and difficult to validate

32 Modular decompositionAnother structural level where sub-systems are decomposed into modulesTwo modular decomposition models coveredAn object model where the system is decomposed into interacting objectsA data-flow model where the system is decomposed into functional modules which transform inputs to outputs. Also known as the pipeline modelIf possible, decisions about concurrency should be delayed until modules are implemented

33 Object modelsStructure the system into a set of loosely coupled objects with well-defined interfacesObject-oriented decomposition is concerned with identifying object classes, their attributes and operationsWhen implemented, objects are created from these classes and some control model used to coordinate object operations

35 Data-flow modelsFunctional transformations process their inputs to produce outputsMay be referred to as a pipe and filter model (as in UNIX shell)Variants of this approach are very common. When transformations are sequential, this is a batch sequential model which is extensively used in data processing systemsNot really suitable for interactive systems

37 Domain-specific architecturesArchitectural models which are specific to some application domainTwo types of domain-specific modelGeneric models which are abstractions from a number of real systems and which encapsulate the principal characteristics of these systemsReference models which are more abstract, idealised model. Provide a means of information about that class of system and of comparing different architecturesGeneric models are usually bottom-up models; Reference models are top-down models

38 Generic modelsCompiler model is a well-known example although other models exist in more specialised application domainsLexical analyserSymbol tableSyntax analyserSyntax treeSemantic analyserCode generatorGeneric compiler model may be organised according to different architectural models

41 Reference architecturesReference models are derived from a study of the application domain rather than from existing systemsMay be used as a basis for system implementation or to compare different systems. It acts as a standard against which systems can be evaluatedOSI model is a layered model for communication systems

43 Key pointsThe software architect is responsible for deriving a structural system model, a control model and a sub-system decomposition modelLarge systems rarely conform to a single architectural modelSystem decomposition models include repository models, client-server models and abstract machine modelsControl models include centralised control and event-driven models

44 Key pointsModular decomposition models include data-flow and object modelsDomain specific architectural models are abstractions over an application domain. They may be constructed by abstracting from existing systems or may be idealised reference models

45 Distributed Systems ArchitecturesArchitectural design for software that executes on more than one processor

47 Distributed systemsVirtually all large computer-based systems are now distributed systemsInformation processing is distributed over several computers rather than confined to a single machineDistributed software engineering is now very important

48 System typesPersonal systems that are not distributed and that are designed to run on a personal computer or workstation.Embedded systems that run on a single processor or on an integrated group of processors.Distributed systems where the system software runs on a loosely integrated group of cooperating processors linked by a network.

52 Distributed systems archiecturesClient-server architecturesDistributed services which are called on by clients. Servers that provide services are treated differently from clients that use servicesDistributed object architecturesNo distinction between clients and servers. Any object on the system may provide and use services from other objects

53 MiddlewareSoftware that manages and supports the different components of a distributed system. In essence, it sits in the middle of the systemMiddleware is usually off-the-shelf rather than specially written softwareExamplesTransaction processing monitorsData convertorsCommunication controllers

54 Multiprocessor architecturesSimplest distributed system modelSystem composed of multiple processes which may (but need not) execute on different processorsArchitectural model of many large real-time systemsDistribution of process to processor may be pre-ordered or may be under the control of a despatcher

56 Client-server architecturesThe application is modelled as a set of services that are provided by servers and a set of clients that use these servicesClients know of servers but servers need not know of clientsClients and servers are logical processesThe mapping of processors to processes is not necessarily 1 : 1

59 Layered application architecturePresentation layerConcerned with presenting the results of a computation to system users and with collecting user inputsApplication processing layerConcerned with providing application specific functionality e.g., in a banking system, banking functions such as open account, close account, etc.Data management layerConcerned with managing the system databases

61 Thin and fat clients Thin-client model Fat-client modelIn a thin-client model, all of the application processing and data management is carried out on the server. The client is simply responsible for running the presentation software.Fat-client modelIn this model, the server is only responsible for data management. The software on the client implements the application logic and the interactions with the system user.

63 Thin client modelUsed when legacy systems are migrated to client server architectures.The legacy system acts as a server in its own right with a graphical interface implemented on a clientA major disadvantage is that it places a heavy processing load on both the server and the network

64 Fat client modelMore processing is delegated to the client as the application processing is locally executedMost suitable for new C/S systems where the capabilities of the client system are known in advanceMore complex than a thin client model especially for management. New versions of the application have to be installed on all clients

66 Three-tier architecturesIn a three-tier architecture, each of the application architecture layers may execute on a separate processorAllows for better performance than a thin-client approach and is simpler to manage than a fat-client approachA more scalable architecture - as demands increase, extra servers can be added

70 Distributed object architecturesThere is no distinction in a distributed object architectures between clients and serversEach distributable entity is an object that provides services to other objects and receives services from other objectsObject communication is through a middleware system called an object request broker (software bus)However, more complex to design than C/S systems

72 Advantages of distributed object architectureIt allows the system designer to delay decisions on where and how services should be providedIt is a very open system architecture that allows new resources to be added to it as requiredThe system is flexible and scaleableIt is possible to reconfigure the system dynamically with objects migrating across the network as required

73 Key points Almost all new large systems are distributed systemsDistributed systems support resource sharing, openness, concurrency, scalability, fault tolerance and transparencyClient-server architectures involve services being delivered by servers to programs operating on clientsUser interface software always runs on the client and data management on the server

74 Key pointsIn a distributed object architecture, there is no distinction between clients and serversDistributed object systems require middleware to handle object communicationsThe CORBA standards are a set of middleware standards that support distributed object architectures